In the description, the concept of superiterative decoder is introduced to differentiate an iteration performed within a decoder according to the prior art and the iteration step in the method according to the invention.
Multiple access communications (TDMA time Divisional multiple access) or communications when the latency must be low generally use short or averagely long codes mentioned above (for example of MPEG size).
Under these conditions, for turbo codes within the general sense of the term, including the turbo codes or TPC Turbo Product Codes, the codes obtained by serial concatenation of convolutional codes or SCCC (the abbreviation standing for Serial Concatenated Convolutional Code), the codes obtained by parallel concatenation of convolutional codes or PCCC (the abbreviation standing for Parallel Concatenated Convolutional Code), the low Density Parity Checks LDPC (the abbreviation standing for Low Density Parity Check), in their binary or m-ary variant, a compromise must be found between complexity of decoding and performance of the code.
The principle of iterative decoding is to generate two items of information of local consistency in two dimensions, that are as independent as possible. Independence is notably ensured between the two dimensions by an interleaver.
The size of the code word limits the decoupling between the two dimensions. A reduced size also amplifies the statistical effect of a poor local coding, which is less well compensated by the remainder of the code.
For this reason, codes of small or medium size are today fairly far from the theoretical limit for reasonable decoding complexities.
For codes of large size (greater for example than 5000 information bits), concatenation procedures are known from the prior art, for example LPCD and turbo code concatenations, for reducing the error floor of the turbo code, due to too low a free distance (within the Hamming sense).
Another possibility consists in not reducing the coding rate of the inner code. The coding rate being smaller, the operating point (expressed in Eb/N0 with Eb energy per bit and N0 spectral density of noise and not of the bits coded by the outer code) is not compensated by the gain of the outer code.
When the system uses small codes, either on account of latency problems related to small throughputs such as in telephony, or on account of granularity problems in dynamic-allocation systems, the coding system using large code words becomes ineffective.